1. Field of the Invention
This invention relates to sensing passive microwave energy and more particularly to passive microwave radiometry apparatus and methods for indicating atmospheric water vapor content, liquid content and temperature.
2. Description of the Prior Art
Microwave radiometers are generally known as passive broad-band receivers that collect radiation emitted from a surface. In the past, microwave radiometers have been designed for response to selected portions of the microwave spectrum according to the function to be performed by the radiometer. For meteorological applications, the 22 GHz region has been identified for determining atmospheric water vapor using the 22.235 GHz water vapor resonance line. 60 GHz is another meteorological region for determining atmospheric temperature profiles based on thermal emission from oxygen.
In the past, the collected signals of a microwave radiometer have been recorded as amplitude-time histories which are related to an absolute temperature reference. As of 1976 temperature sensitivities of .+-.0.05 degrees Kelvin (herein noted as "K") rms were said to be possible, but it was reported that due to size, weight and power limitations, compromises were necessary in applying microwave radiometers to particular applications. Such applications of passive microwave radiometers include measuring total water vapor content, liquid water content and refractive index properties of the atmosphere.
These quantities are typically inferred from passive microwave measurements of the sky brightness temperature at a plurality of microwave and millimeter wave frequencies having different water vapor and liquid water absorption coefficients. Typical "dual-channel" systems use two frequencies, one at about 22.4 GHz and the other at about 31.4 GHz. The 22.4 GHz frequency corresponds to the peak of a broad water vapor absorption line and 31.4 GHz frequency is near a relative minimum in the water vapor absorption spectrum. At these frequencies, absorption and re-radiation due to liquid water aerosols obey Rayleigh's scattering theory. Their attenuation (dB/Km) thus varies as the inverse square of the wavelength in this spectral region. Absorption is stronger in the 31.4 GHz region than at the 22.4 GHz region if liquid water is present. Ice particles are effectively transparent at both frequencies, whereas water vapor absorbs more strongly near 22.4 GHz than at 31.4 GHz.
The microwave absorption coefficients of water vapor and liquid water clouds vary as a function of frequency in this spectral range. The attenuation at 22.4 GHz caused by a fixed amount of water vapor (e.g. 1 gram per cubic meter over a 1 kilometer path length) increases with increasing altitude. However, at certain frequencies above and below 22.4 GHz such attenuation decreases with increasing altitude. As a result, the attenuation curves (attenuation vs. frequency) for different altitudes cross each other at a pressure invariant frequency on each side of 22.4 GHZ. Such pressure invariant frequencies are at about 20.6 GHz and about 24.1 GHz. These crossing points can be referred to as pressure invariant or crossover points on the attenuation vs. frequency curves. The attenuation caused by a fixed amount of cloud liquid water droplets (e.g. 0.1 gram per cubic meter over a 1 kilometer path length) does not peak in the same spectral range, but does vary with changing altitude.
In their Journal of Climate and Applied Meteorology article at Vol. 22: pp. 789-806, May, 1983, entitled "A Steerable Dual-Channel microwave Radiometer for Measurement of Water, Vapor and Liquid in the Troposphere", D. C. Hogg and others ("Hogg B") selected two frequencies, 20.6 GHz and 31.6 GHz, for water vapor and liquid measurements. A separate radiometer was used for each frequency and they were located in a single package that provided an essentially constant temperature environment for the radiometers and the antenna.
The oxygen microwave absorption band in the 60 GHz region of the spectrum was discussed in the same issue of the Journal of Climate and Applied Meteorology.
In an article entitled "An Automatic Profiler of the Temperature, Wind and Humidity in the Troposphere", at pp 807-831, Hogg and others ("Hogg B") discuss the use of this band to measure the temperature profile of the atmosphere. Since oxygen is a well mixed gas of known concentration, changes in oxygen band sky brightness indicate different temperatures of the atmosphere above the antenna. Multiple oxygen band wavelengths and/or multiple antenna look angles allow temperature profiles to be retrieved since they have different attenuation and thus different spatial weightings of radiating gas temperature.
Known experimental prototypes using known methods to measure true "sky brightness temperature" are subject to drift in gain and offset are expensive power consumptive, and not easily portable. For example in Hogg B a heat pump is used to stabilize the temperature of a housing of a trailer that contains the radiometer system. Similarly, a building is shown in Hogg A for housing the radiometers, radar receivers and other equipment. Also, in an article by M. A Janssen entitled "A New Instrument For The Determination of Radio Path Delay Due To Atmospheric Water Vapor", 1985, IEEE Transactions on Geoscience and Remote Sensing. GE-23:485-490, a smaller radiometer instrument is shown, but a thermoelectric heat pump is used to maintain the radiometer at a constant temperature.
Differentials between the radiometer temperature and the temperature of the sensed atmosphere present problems to the radiometer designer. On the one hand, sky brightness temperatures typically range from 20K to 150K in the 20 to 35 GHz band. On the other hand, the physical temperature of the antenna and the waveguides of a ground-based radiometer are nominally 300K. Present calibration techniques require frequent "tipping curve" calibrations that use the cosmic background (2.7K) as a calibration standard. As a result, full realtime calibration of the available radiometers is not feasible with present techniques when clouds are present, which is the very condition of greatest meteorological interest.
If all physical radiometer components are ideal and have neither resistive losses, impedance mismatch, nor thermal self re-radiation of microwave energy, then the transfer function of sky radiance signal power into the receiver is unity. A typical microwave water vapor radiometer-received sky brightness temperature (power) of 50K [T.sub.B,sky ] would appear at the radiometer output as an identical effective output brightness temperature of 50K [T.sub.B,out ]. However, the actual situation is more complex. In particular, DC signal offset appears in radiometers since they use non-ideal microwave antennas and transmission components. For example, where the radiometer components are all ideal except for the microwave horn antenna itself, an antenna dissipative loss of only 0.1 dB corresponds to an antenna fractional loss of [10..sup.1/10 -1=0.023], or a horn efficiency [E.sub.horn ] of 97.7%. Under these conditions, the Rayleigh-Jeans Approximation and Schwarzchild's Equation can be used to calculate the effective output brightness temperature of the radiometer as: EQU T.sub.B,out =[T.sub.B,sky ][E.sub.horn ]+T.sub.horn [1-E.sub.horn ]Eq (1)
If typical values are assumed for:
T.sub.B,sky =50K (for a water vapor radiometer at 23.8 GHz), PA1 T.sub.horn =300K (local ambient temperature), and PA1 E.sub.horn =0.977 (0.1 dB losses); PA1 L .sub.f =0.1 dB (feedhorn losses) PA1 L.sub.wg =0.2 dB (waveguide losses) PA1 L.sub.sw =0.3 dB (modulator switch losses) PA1 L.sub.i =0.15 dB (isolator losses)
then, after applying the radiometric transfer function of Eq. 1, the result is: EQU T.sub.B, out =55.75K
Since this is an overestimate of +5.75K, this offset error is already ten times the amount of a desirable design goal (absolute accuracy level) of 0.5K in sky brightness temperature and appears as a DC offset level in measured sky brightness temperature. The gain drift is -2.3% in this example.
Unfortunately, actual transfer function system offsets are likely to be somewhat worse than the 0.1 dB losses noted above. For example, Stacey, in Spaceborne Receivers. Basic Principles. JPL Publication 84-89, Dec. 1, 1984, gives the following breakdown for 0.75 dB in hypothetical radiometer total internal losses:
Repeating the analysis of Eq. 1 with such 0.75 dB internal losses under the same conditions indicates that a sky brightness temperature of 50K would be sensed at the radiometer output as 97.125K. To keep this offset error within the design goal of 0.5K the above analysis indicates that the front end antenna losses, according to Eq. 1, can be no more than 0.2% or 0.008 dB.
Prior physical hardware are not this accurate. For example, Stacey, in Microwave Blackbodies for Spaceborne Receivers, JPL Publication 85-10, 1985), found that dissipative losses in high quality corrugated horn antennas are about 0.2 dB. Wheeler, in his book Introduction to Microwaves, (Prentice Hall, 1963), gave theoretical formulas for dissipative losses in copper waveguide. For size WR34 rectangular waveguide at an operating frequency of 23.8 GHz, a theoretically ideal Waveguide would have dissipative losses of 0.144 dB/foot. Thus, by the above analysis, 0.7 inches of waveguide at 300K would cause a 0.5K sky brightness error. Similar tiny loss mismatch in waveguide switches will destroy absolute accuracy in switched hot-loads in the waveguide path. Non-dissipative voltage standing wave ratio (VSWR) mismatch in waveguide paths also destroys absolute accuracy.
In summary, prior art passive radiometers must be calibrated at frequent intervals using the tipping curve technique, even though such technique depends on clear skies for accuracy. Further, based on these calculations using the characteristics of prior art passive radiometers, it appears that switched hot-loads in the waveguide path do not give an accurate absolute calibration by themselves. Nonetheless, the prior art regards switched hot-loads as absolute brightness temperature sources referenced to the antenna input since their absolute temperature is known. In reality, in applicants experiences, lossy microwave components cause both gain and offset drift of the transfer function. Finally, to avoid drift of the transfer function the Prior art radiometers consume power necessary to maintain them at a constant temperature.